1. Field of the Invention
The present invention relates to treatment systems for clarification of a fluid stream and, more particularly, to buoyant media introduced into the flow to induce a rising force on particles to be removed from the fluid.
2. Background of the Invention
Technologies for the separation of components of a fluid stream on the basis of density are well known in the art. The simplest systems rely on differences in densities alone to accomplish this separation, with heavier components settling, in the absence of agitation, to the bottom of the fluid for removal.
For small particles, the time required for the unassisted settling process can be unacceptably long. Stokes' Law predicts that spherical particles suspended in a fluid medium settle at a rate proportional to the second power of the particle diameter. Thus, large particles will settle much faster than smaller ones. To increase the rate of settling, coagulants can be used. A coagulant is a substance that produces a loosely-massed precipitate. As the precipitate forms, it entraps solid particles contained in the fluid. Aluminum sulfate and ferric sulfate are among the materials used in the art as coagulants.
Particles in solution often have a net surface charge. As a result, they tend to repel each other and resist the formation of agglomerates. Coagulation is the process whereby these repulsive charges are neutralized. After charge neutralization, the particles begin to collect into larger aggregates. This agglomeration process is known as flocculation; the resulting aggregate is known as a floc. Flocculant aids are thus effective as solid-liquid separating agents. Flocculant aids are typically synthetic water soluble polymers based on acrylamide. There are some natural occurring polymers, such as chitosan, that can be used for this purpose.
Entrainment can be used in conjunction with flocculation to expedite the separation process. For example, grains of sand can be introduced to the fluid to be treated. The floc that forms entraps the grains of sand, and the density of the floc is increased as is its settling speed. Alternatively, bubbles may be generated as the floc is formed, and their entrainment produces a floc that rises to the surface of the fluid rather than settling to the bottom. However, additional equipment and operating expenses are required to generate the bubbles. Other settling media may be selected on the basis of specific gravity, particle size, surface properties, or chemical properties. Increasing the concentration of these settling media improves the efficiency of the flocculation process, because more particle collisions result and the size of the resulting agglomerated particles increases more rapidly. Also, the settling capacity of the system increases with concentration. However, increasing the concentration of entrained materials increases the cost of the separation process, and increases the volume of the waste materials produced. For settling media with specialized properties, these cost constraints can be significant.
Solids separation processes employing settling inherently require energy in the flocculation stage, produce large volumes of sludge, produce effluent with small, difficult to settle particles, and require large, complex process vessels. These characteristics lead to increased costs. Accordingly, a need exists to reduce the costs associated with solids separation in high rate fluid clarification systems.